kvcache: Enable SWA to retain additional entries

Models that use sliding window attention can only resume a sequence
from the cache if it falls within the saved windows. This works well
if the next message picks up where the old one left off. However, it
generally prevents a partial prefix match unless the entire conversation
falls within the sliding window.

This can be a problem with reasoning models where the traces are
supposed to be removed from future messages, forcing the entire
history to be re-evaluated.

This change allows models to specify that a larger amount of the
history be retained in memory, to allow more partial resumption.
It still respects the window that the model was trained on for
token generation.

kvcache/causal_test.go

@@ -60,6 +60,8 @@ func TestSWA(t *testing.T) {
 
 	cache.Init(backend, ml.DTypeF16, 1, 16, 16)
 
+	x := float32(math.Inf(-1))
+
 	tests := []testCase{
 		{
 			name:          "FirstBatch",
@@ -69,7 +71,12 @@ func TestSWA(t *testing.T) {
 			pos:           []int32{0, 1, 2, 3},
 			expected:      []float32{1, 2, 3, 4},
 			expectedShape: []int{1, 1, 4},
-			expectedMask:  []float32{0, float32(math.Inf(-1)), float32(math.Inf(-1)), float32(math.Inf(-1)), 0, 0, float32(math.Inf(-1)), float32(math.Inf(-1)), float32(math.Inf(-1)), 0, 0, float32(math.Inf(-1)), float32(math.Inf(-1)), float32(math.Inf(-1)), 0, 0},
+			expectedMask: []float32{
+				0, x, x, x,
+				0, 0, x, x,
+				x, 0, 0, x,
+				x, x, 0, 0,
+			},
 		},
 		{
 			name:          "SecondBatch",
@@ -79,7 +86,53 @@ func TestSWA(t *testing.T) {
 			pos:           []int32{4, 5},
 			expected:      []float32{5, 6, 3, 4},
 			expectedShape: []int{1, 1, 4},
-			expectedMask:  []float32{0, float32(math.Inf(-1)), float32(math.Inf(-1)), 0, 0, 0, float32(math.Inf(-1)), float32(math.Inf(-1))},
+			expectedMask: []float32{
+				0, x, x, 0,
+				0, 0, x, x,
+			},
+		},
+	}
+
+	testCache(t, backend, cache, tests)
+}
+
+func TestSWAMem(t *testing.T) {
+	backend := &testBackend{}
+	cache := NewSWAMemCache(1, 3, nil)
+	defer cache.Close()
+
+	cache.Init(backend, ml.DTypeF16, 1, 16, 16)
+
+	x := float32(math.Inf(-1))
+
+	tests := []testCase{
+		{
+			name:          "FirstBatch",
+			in:            []float32{1, 2, 3, 4},
+			inShape:       []int{1, 1, 4},
+			seqs:          []int{0, 0, 0, 0},
+			pos:           []int32{0, 1, 2, 3},
+			expected:      []float32{1, 2, 3, 4},
+			expectedShape: []int{1, 1, 4},
+			expectedMask: []float32{
+				0, x, x, x,
+				0, 0, x, x,
+				x, 0, 0, x,
+				x, x, 0, 0,
+			},
+		},
+		{
+			name:          "SecondBatch",
+			in:            []float32{5, 6},
+			inShape:       []int{1, 1, 2},
+			seqs:          []int{0, 0},
+			pos:           []int32{4, 5},
+			expected:      []float32{4, 5, 6},
+			expectedShape: []int{1, 1, 3},
+			expectedMask: []float32{
+				0, 0, x,
+				x, 0, 0,
+			},
 		},
 	}
 
@@ -437,6 +490,70 @@ func TestCanResume(t *testing.T) {
 	}
 }
 
+func TestCanResumeSWAMem(t *testing.T) {
+	backend := &testBackend{}
+	windowSize := int32(4)
+	memSize := int32(5)
+	cache := NewSWAMemCache(windowSize, memSize, nil)
+	defer cache.Close()
+
+	cache.Init(backend, ml.DTypeF16, 1, 16, 16)
+
+	context := backend.NewContext()
+	defer context.Close()
+
+	err := cache.StartForward(context, input.Batch{
+		Positions: []int32{0, 1, 2, 3, 4, 5},
+		Sequences: []int{0, 0, 0, 0, 0, 0},
+	}, false)
+	if err != nil {
+		t.Fatalf("StartForward failed: %v", err)
+	}
+
+	cache.SetLayer(0)
+	tensor := context.FromFloatSlice([]float32{1, 2, 3, 4, 5, 6}, 1, 1, 6)
+	cache.Put(context, tensor, tensor)
+
+	// shift window by adding position 6
+	err = cache.StartForward(context, input.Batch{
+		Positions: []int32{6, 7},
+		Sequences: []int{0, 0},
+	}, false)
+	if err != nil {
+		t.Fatalf("StartForward failed: %v", err)
+	}
+
+	cache.SetLayer(0)
+	tensor = context.FromFloatSlice([]float32{7, 8}, 1, 1, 2)
+	cache.Put(context, tensor, tensor)
+
+	// only the latest position has overlapping windows
+	if cache.CanResume(0, 0) {
+		t.Errorf("after shift: CanResume(0, 0) = true, want false (outside window)")
+	}
+	if cache.CanResume(0, 1) {
+		t.Errorf("after shift: CanResume(0, 1) = true, want false (outside window)")
+	}
+	if cache.CanResume(0, 2) {
+		t.Errorf("after shift: CanResume(0, 2) = true, want false (outside window)")
+	}
+	if cache.CanResume(0, 3) {
+		t.Errorf("after shift: CanResume(0, 3) = true, want false (outside window)")
+	}
+	if cache.CanResume(0, 4) {
+		t.Errorf("after shift: CanResume(0, 4) = true, want false (outside window)")
+	}
+	if cache.CanResume(0, 5) {
+		t.Errorf("after shift: CanResume(0, 5) = true, want false (outside window)")
+	}
+	if !cache.CanResume(0, 6) {
+		t.Errorf("after shift: CanResume(0, 6) = false, want true (inside window)")
+	}
+	if !cache.CanResume(0, 7) {
+		t.Errorf("after shift: CanResume(0, 7) = false, want true (latest position)")
+	}
+}
+
 type testBackend struct {
 	ml.Backend
 }